Molded dielectric layer in print-patterned electronic circuits

ABSTRACT

A method forms a first active electronic layer, prints an array of pillars on the first active electronic layer, dispenses a curable polymer over the array of pillars, molds the curable polymer by contacting the curable polymer with a mold structure to displace the curable polymer from upper surfaces of the pillars, cures the curable polymer to produce a hardened polymer, and removes the array of pillars to leave an array of holes in the hardened polymer. Another method provides a substrate having selected areas, prints an array of pillars on the substrate, dispenses a curable polymer over the array of pillars, molds the curable polymer by contacting the array of pillars with a mold structure to displace the curable polymer from upper surfaces of the pillars, cures the curable polymer to produce a hardened polymer, and removes the array of pillars to leave an array of holes in the hardened polymer corresponding to the selected areas. Another method forms a first active electronic layer on a substrate, prints an array of conductive pillars on the active electronic layer on a substrate, dispenses a curable polymer on the array of conductive pillars, molds the curable polymer by contacting the array of pillars with a mold structure to displace the curable polymer from the upper surfaces of the conductive pillars, curing the curable polymer to produce a hardened polymer, and forms a second active electronic layer on the hardened polymer such that the second active electronic layer is in electrical connection with the first active electronic layer through the conductive pillars.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 11/615,229, entitled MOLDED DIELECTRIC LAYER IN PRINT-PATTERNEDELECTRONIC CIRCUITS, filed Dec. 22, 2006, the disclosure of which isherein incorporated by the reference in its entirety.

GOVERNMENT FUNDING

This invention was made with Government support under CooperativeAgreement No. 70NANB3H3029 awarded by the National Institute ofStandards and Technology. The Government has certain rights in thisinvention.

BACKGROUND

It is possible to form electronic circuits using printing technologies,typically ink-jet printing, where the ‘ink’ is actually liquids that canform the structures. Printed lines and dots used in forming the circuithave relatively large features sizes that are not conducive to someelectronic devices.

In one example, assume an active matrix display backplane with highfill-factor pixels. Fill factor is generally a ration of the area ofpixel that is actively controlling, or in the case of active-matrixsensor arrays receiving/sensing light, to the area of the entire pixel.Each pixel of a thin-film transistor (TFT) active matrix backplanegenerally has a switching transistor and a pixel pad for each pixel.

Each transistor typically consists of several layers: a gate layer, agate dielectric layer, a source/drain layer and a semiconducting layer.In typical pixels with bottom-gate TFTs, the data lines which apply thedata signals to the pixels are on the same level as the source/drainlayer of the TFT and of the pixel pad. If the data lines are wide, theregion of the pixel pad becomes smaller within a given area for a pixel.The area of the pixel is limited because image quality generally comesfrom a number of pixels per image, and as many pixels as possible aresqueezed onto a given backplane.

In an approach to overcome this problem an additional metal layer isintroduced which extends or ‘mushrooms’ the pixel drain pad layer overthe transistor circuitry and partially over the data lines. This alsoshields the TFT channel region from light which is essential for lowcharge-leakage in the TFT off-state. Vias are formed in a dielectriclayer over the transistor to allow connection between this ‘mushroommetal’ pixel pad and the underlying drain pad which is connected to thedrain of the TFT. The example of a display backplane is compelling, butforming of vias for interconnects between layers is also important inother electronic circuits where often multiple dielectric layersseparate conducting, semiconducting or otherwise functional layers,referred to here as active electronic layers or active layers. Moreover,forming simple via holes in a dielectric layer is required for otherapplications such as in microfluidic circuits where the dielectric layermay contain a fluid and sensing elements lie in a layer underneath thedielectric.

However, forming vias in a printing technology can be a problem.Printing technologies tend to be additive, where things are addedtogether to form images, such as in color printing where colors areadded together to form a final color. Circuits formed from printingtechnologies are generally formed by adding layers to other layers toform the structures. In one example, a conventional semiconductorfabrication process deposits a continuous dielectric layer. To formvias, the process must etch the vias into the dielectric. In anotherexample, a micromolding process molds a polymeric dielectric layer withvias in a single step, if alignment is included in the step. However, athin surface layer typically remains at the bottoms of the vias that hasto be removed by etching.

Generally, etching, such as wet-chemical or plasma etching, does notoccur in printing technologies. This makes forming the vias problematic,as vias are normally formed by etching.

SUMMARY

An embodiment is a method that forms a first active electronic layer,prints an array of pillars on the first active electronic layer,dispenses a curable polymer over the array of pillars, molds the curablepolymer by contacting the curable polymer with a mold structure todisplace the curable polymer from upper surfaces of the pillars, curesthe curable polymer to produce a hardened polymer, and removes the arrayof pillars to leave an array of holes in the hardened polymer.

Another embodiment is a method that provides a substrate having selectedareas, prints an array of pillars on the substrate, dispenses a curablepolymer over the array of pillars, molds the curable polymer bycontacting the array of pillars with a mold structure to displace thecurable polymer from upper surfaces of the pillars, cures the curablepolymer to produce a hardened polymer, and removes the array of pillarsto leave an array of holes in the hardened polymer corresponding to theselected areas.

Another embodiment is a method that forms a first active electroniclayer on a substrate, prints an array of conductive pillars on theactive electronic layer on a substrate, dispenses a curable polymer onthe array of conductive pillars, molds the curable polymer by contactingthe array of pillars with a mold structure to displace the curablepolymer from the upper surfaces of the conductive pillars, curing thecurable polymer to produce a hardened polymer, and forms a second activeelectronic layer on the hardened polymer such that the second activeelectronic layer is in electrical connection with the first activeelectronic layer through the conductive pillars.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention may be best understood by reading thedisclosure with reference to the drawings, wherein:

FIG. 1 shows an example of an array of pillars.

FIG. 2 shows a more detailed view of a pillar.

FIGS. 3-8 show an example of a method of forming vias using printingtechniques.

FIGS. 9-11 show an alternative example of a method of forming vias usingprinting techniques.

FIGS. 12-15 show an example of a method of forming extended regions of adielectric layer.

FIGS. 16-20 show examples of alternative methods of forming vias usingprinting techniques.

FIGS. 21-24 show an example of a method of forming via contacts usingprinting techniques.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows an example of sacrificial pillars printed on a substrate.These pillars such as 10 define a via area in which will be formed a viacontact. Wax, such as Kemamide-based wax is just one possible materialthat could be used to form the pillars. Waxes have a low viscosity abovetheir melting point and therefore are suitable for jet-printing. Belowtheir melting point they solidify. Since these materials are printedwithout solvents, the volume shrinkage is relatively low which enablesthe printing of rather tall structures. Other materials includesolvent-based polymers such as ProLift™ manufactured by Brewer Scienceor heat decomposable polymers such as the family of Unity™ polymersmanufactured by Promerus. Except for the heat decomposable polymer, thematerials would be later removed in a solvent that would not attack thesubsequently applied dielectric, typically a polymer. ProLift™ is aslift-off polymer which is rather temperature stable and resistant toorganic solvents, but it retains solubility in aqueous developers. Inthe case of the heat decomposable material, the material would beremoved at a later stage of the process by heating beyond adecomposition temperature. While much of the discussion here may focuson individual pillars, the pillars may also consist of walls or othergeometries such as linear structures, rectangular structures, extendedcircular structures, etc. that later form reservoirs, channels orregions.

FIG. 2 shows an example of a pillar structure formed by printing.Ink-jet printing may be one example printing method, but other printingmethods may also be chosen, such as dip-pen deposition, flexographicprinting, screen printing or other deposition methods that are capableof selectively depositing small quantities of a material. Examples ofink-jet printing include piezo-ink-jet printing, thermal ink-jetprinting, electrostatic ink-jet printing, etc. To achieve a verticalextent higher than a single drop of ink-jetted material, the pillars maybe formed of a stack of drops of ink. FIG. 2 shows an example of apillar formed from three drops of the ink, 2, 4 and 6. It must be notedthat as the term ‘ink’ is used here, it includes any liquid dispensedusing printing technologies for the formation of electronic circuits andcomponents and microstructures.

FIG. 3 shows the beginning of a process to form vias using printingprocesses, rather than conventional processes employing etching. A viais a hole or opening in a layer that allows connections between layersabove and below the layer in which the via exists. Vias may be leftempty to expose a selected area on the underlying substrate, they may befilled or partially filled with metal, either before or after formationand they may be also filled or partially filled with a semiconductingmaterial or other functional material. Examples of the selected areas onthe substrate may be areas in which a sensor or other sensitive materialresides, or an actuator for which it is desirable to leave the area opento the surrounding environment. Selected areas are defined regions onthe substrate for which it is desirable that they be accessible fromhigher layers. The sensor may be a heat sensor such as a thermistor, itmay be a pressure sensor or a flow sensor or a chemical or biosensorsuch as an ion sensor, pH sensor or surface acoustic wave sensor, forexample. As an actuator, a heater may be located on the substrate thatheats a fluid above. Alternatively, a membrane actuator or an ultrasonicactuator may be located on the substrate, for example. Vias or holesfilled or partially filled or coated with metal or other conductivematerial may be referred to here as via contacts or conductive paths.

In FIG. 3, a printing process deposits or otherwise forms an array ofsacrificial pillars such as 10 on a substrate 18. As discussed above,the printing process may repeat as needed to achieve the proper heightfor the pillars. In one example the printing process is a jet-printingprocess and the deposited material is a hot-melt wax.

In FIG. 4, a curable polymer 12 covers the pillars. It is dispensed byany known dispensing methods for liquids or flowable substances,including pipette dispensing, syringe dispensing, other pressure drivendispensing methods, including extrusion-type dispensing, but also spraydispensing or simple pouring from a reservoir, etc. A curable polymer isa substance that is a liquid or flowable substance/polymer until itundergoes a curing process that causes it to harden. The polymermaterial may include organic polymers, inorganic polymers,organic-inorganic hybrids and composites, for example.

The curing process may involve the application of heat or radiation,including UV light. The curing process may also be a catalytic curingprocess. At the point of the process shown by FIG. 4, the curablepolymer remains in its moldable, uncured state. Example polymers are theUV curable polymers Norland optical adhesive 68 or Norland opticaladhesive 60 (Norland Products, Inc. Cranbury, N.J.) and the twocomponent epoxy Devcon 5 Minute Epoxy (ITW Performance Polymers, RivieraBeach, Fla.). If the pillar structures are relatively heat resistant,such as for the later mentioned metal pillars, or if the pillars have arelatively high melting/softening point, the curable polymer may also bea thermoplastic polymer. Here ‘curing’ does not refer to permanentcuring, but it would be used in the context of ‘hardening’ orsolidifying by lowering the process temperature. Example polymers wouldbe thermoplastic polymers often used in molding applications such asPMMA (polymethylmethacrylate), PC (polycarbonate), PSU (polysulfone),COC (cyclo-olefine copolymer), PS (polystyrene), etc.

In FIG. 5, the process applies a mold 14 to the uncured polymer. Themold may consist of a silicone elastomer, such as Sylgard 184 siliconemanufactured by Dow, Gel-Film® (Gel-Pak) materials, etc. The moldmaterial may be an elastomeric material or it may be a rigid materialand it may be coated with a low-surface-energy coating such as afluorocarbon coating or it may be treated with commonly knownmold-release agents. During the molding process, the surface of the moldcontacts the top surfaces of the pillars and displaces the curablepolymer.

In the case of an elastomeric mold the contact between the pillar topsurface and the mold may be increased due to the deformation of theelastomer. For larger arrays of pillars, this can make the displacementof the polymer on all the pillars more reliable. In the case ofthermoplastic pillars or wax pillars, one may also heat the substrate atthis point to cause the pillars to soften, which in combination with aslight pressure from the mold may result in an improved via profile dueto the occurring deformation in the pillars. Softening the pillars mayalso assist in displacing the curable polymer from the tops of thepillars.

FIG. 5 also shows the application of UV light, assuming that the curablepolymer is a UV curable polymer, just one example of many differenttypes of curable polymers suitable for this process. The application ofthe UV light causes the curable polymer in this example to ‘cure’ orharden. In another example, UV exposure may not be required and thepolymer may cure by catalytic curing such as in a two-component epoxypolymer, for example. The polymer 12 may also contain inorganicparticles such as titania or barium titanate nanoparticles if anincreased dielectric constant is desired. FIG. 6 shows the resultingstructure after removal of the mold. The hardened polymer has formed alayer having the top surfaces of the array of pillars exposed.

It must be noted that using a rigid mold instead of an elastomeric mold,or if the pillars soften after heating, the pillars would not extendfrom the surface in FIG. 6. The extending pillars shown in FIG. 6 areseen if one uses a soft elastomer mold because it deforms slightlyaround the pillars.

FIG. 7 shows the hardened polymer after removal of the pillars. In theexample of a Kemamide-wax pillar, the pillars would dissolve in a warmisopropanol solution. The solvent should not attack the curable polymer,which could be one of the polymers mentioned above. The hardened polymerhas an array of holes or openings 20, which may have any shape.

If desired, a conductive layer 22 may form electrical contacts in theopenings, between a first functional layer on the substrate and a secondfunctional layer on the polymer layer. FIG. 8 shows an example of aconductive layer. The conductive layer may consist of a patterned metal,a polymeric conductor, a semiconductor or other conducting orsemiconducting material. The material may be deposited for example froma solution by jet-printing or by conventional vacuum deposition methods.In one example, the conducting material is jet-printed silver depositedfrom a solution of silver nanoparticles. The holes or openings in thepolymer resulting from the molding process may have different profiles,depending upon the molding techniques. The openings may tilt, or have anenhanced profile with a decreasing diameter from bottom to top. Thesealternatives come from variations in the printing process such asaltering the diameters of the drops, or moving a second drop slightly toone side of a first drop. The printing process discussed up to now hasnot mentioned any alignment.

FIG. 9 shows an example of a structure having a first functional layerfor which the process should include an alignment process. This examplehas a transistor structure having source 26 and drain 24 contacts on thefunctional layer on the substrate. The process would form the polymerlayer over the source and drain contacts, with the openings aligned tothe source and drain contacts. Functional or active layers may includecontacts, as in the source and drain contacts, active areas, such asimplanted, semiconductive regions, conductive traces, components, etc.

In FIG. 10, the process deposits a semiconductive material 28 into theopenings over the source and drain. This material forms the channel ofthe thin-film transistor structure. In the shown example thesemiconductor material is deposited from solution such as by ink-jetprinting. Many solution processable semiconductors are available,including polymeric semiconductors and semiconductor precursors. In oneexample, the semiconductor is solution deposited polythiophene PQT-12(poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene]). Here, thefunction of the via is to contain the semiconductor solution and toprevent excessive spreading. The described transistor structure may be abottom gate transistor in which case the gate electrode lies on thesubstrate below the source-drain contacts and below a gate dielectric.An insulating layer 29 may be deposited above the semiconductor, whichalso acts as a barrier layer against moisture and air.

The transistor structure may also be a top-gate transistor in which casethe gate is located above the semiconductor within the via. A gatedielectric is deposited above the semiconductor. Deposition may occur byjet-printing of the dielectric material from solution or thesemiconductor and dielectric may be deposited from a blended solution ofboth materials. In the latter process, the dielectric material phaseseparates on top of the semiconductor during the drying process. In oneexample, the polythiophene PQT-12 and the polymer PMMA(Poly-methyl-methacrylate) form such a polymer blend that phaseseparates.

In the case of a top-gate transistor, the structures shown in FIG. 10may be used as ISFET (ion selective field effect transistor) structuresin which a liquid fills the vias which act as a reservoir. The metalgate of a conventional field effect transistor is replaced by areference electrode located somewhere in the liquid.

In a conventional top-gate field effect transistor a metal depositedinto the via will form the gate. Shown in FIG. 11 is the cross-sectionof a bottom gate thin-film transistor with ‘mushroom-metal’ thatcontacts the drain pad. The conductive layer 30 in FIG. 11 establishescontact to the drain of a transistor and it extends over the transistorarea thereby shielding the channel area from ambient light. Thedielectric layer above the semiconductor in FIG. 11 is deposited thickenough so that the ‘mushroom metal’ does not have any gating effect onthe transistor channel.

Other layers such as for diode structures may be deposited and patternedonto the conductive layer 30, thus turning the pixels into lightsensors. In this manner, an electrical connection forms between thefunctional layer on the substrate and a subsequent second functionallayer, such as a pixel pad in a display or image sensor back plane,through the openings formed from the sacrificial pillars.

In addition to the openings, the polymer may have formed in it extendedregions. Referring to FIG. 12, one can see that the mold 14 differs inits structure now, having relief regions such as 34. When the mold 14comes into contact with the curable polymer in its soft or liquid state,application of pressure causes the polymer to fill the relief region.After hardening and removal of the sacrificial pillars, the hardenedlayer has extended regions such as 36, shown in FIG. 13. This structuremay provide microfluidic channels, wells or reservoirs with vias to thesubstrate. The vias may expose sensing circuits, actuators or heaters tomanipulate the fluid or to sense properties of the fluid or to detectcomponents of the fluid. As mentioned earlier, the via openings may alsobe elongated openings, for example patterned along the channel length,so that the fluid inside the channels can be sensed or actuated bycircuitry on the substrate all the time during its flow within thechannel.

FIG. 14 shows filling of the vias with a conductive material orotherwise functional material, the vias being filled either fully suchas 38 or partially, such as 39. Whether a via becomes fully or partiallyfilled depends upon the application for which the structure is intended,as well as the materials and process used. The fully or partially filledvias form conductive paths.

One application of the extended regions includes using the extendedregions to form at least one reservoir. As shown in FIG. 15, a reservoir40 forms between two of the extended regions and the lid 41. The viacontacts may provide electrical signals to the region defined by thereservoir to cause a reaction in the material to achieve a particulareffect, depending upon the application or electrical signals may be readout through the vias.

These processes would fill the reservoirs with different materials. Forexample, for sensor applications, a charge generation material mayreside in the space between the reservoirs and the lid, chargegeneration materials may include lead-iodide and mercury-iodide.Typically for display applications, materials in the reservoirs includeelectrophoretic ink, liquid crystals, electrochromic materials orelectrowetting display fluids. The reservoirs may also form liquid orgas microchannels.

Having presented a general discussion of the formation of a moldedpolymer or molded dielectric layer, various alternatives andmodifications become available. For example, FIG. 16 shows severalspacer beads such as 42, on the substrate between the sacrificialpillars. Example materials are glass spheres, polystyrene spheres orglass fiber spacers. In one particular example, the spacers are 5 micronfiber spacers from EM Industries of Hawthorne, N.J., and they aretypically employed as spacers in the fabrication of liquid crystaldisplays. The spacer material may be chosen based upon the desiredthickness of the dielectric layer. The spacer beads serve to prevent themolding process from compressing the curable polymer excessively and toassure a uniform thickness of the molded dielectric layer over a largearea. The spacer beads may be deposited by spraying or by inkjetprinting from a dispersion and subsequent drying of the solvent.

In an alternative process, the curable polymer 12 covers the pillars andthe spacer beads, as shown in FIG. 17. Instead of applying the spacerbeads first, they may have been dispersed in the polymer. In the moldingprocess of FIG. 18, a mold having relief regions is shown. It must benoted that the mold may or may not have relief regions as used in thisprocess; it is one option to be considered. FIG. 18 shows the option ofheating the substrate to soften the thermoplastic polymer pillars priorto compression by the mold 14. For purposes of this discussion,thermoplastic polymer materials includes waxes. The spacer beads ensurethat the mold does not compress too much, over-flattening the pillarsfor any particular application.

The flattened pillars may result in an improved via profile. The exactvia profile also may depend upon the wetting properties of the mold bythe wax, if it becomes liquid. Good wetting properties typically resultin a small contact angle and a via that is wider towards the top. Anexample of the resulting via 46 is shown in FIG. 20, after the curingprocess of FIG. 19. Vias with a greater diameter near the top areadvantageous for contact metallization or contact coating.

As an alternative to the pillar removal process, a fully additiveprocess may simplify making via contacts between the first and secondlayers. As shown in FIG. 21, rather than depositing sacrificial pillars,the process deposits metal or conductive pillars such as 50 onto thesubstrate. The dispensing of the curable polymer 12 in FIG. 22 and theapplication of the mold 14 of FIG. 23 are similar to those processesalready discussed. The resulting polymer layer in FIG. 24 still hasopenings, but the openings now accommodate the metal pillars 50.

One issue that may arise in formation of the pillars concerns the heightof the pillars. Higher pillars may be desired that are not easily formedfrom repeated jetting of ink drops in a stack. FIG. 25 shows pillar 50and alternative structures. The metal pillars may include metalparticles, such as silver nanoparticles, resulting in a metal pillar 50.To achieve higher pillars, the silver nanoparticles may be mixed withother dispersible larger particles such as styrene particles, glassspheres, etc., shown as 52 in FIG. 25. These particles may havedimensions on the order of micrometers or tens of micrometers and willbe referred to here as ‘structural particles.’ The silver nanoparticlesmay form a conductive layer around the structural particles. The silvernanoparticles may be first mixed with those structural particles andthen the solution is dispensed. Alternatively, the structural particlesmay be first dispensed and the conductive material is then deposited orprinted on top.

Yet another alternative involves depositing a relatively tall polymerbump and then coating the bump with a layer of conductor, as shown by54. Both the bump and the coating would result from printing processes.The polymer pump may be jet-printed UV-curable polymer and the conductormay be a jet-printed layer of silver nanoparticles or conductive polymersuch as PEDOT:PSS (Baytron P).

Another possible concern with this process arises with regard to the topsurfaces of the pillars. Any residue of the curable polymer on the topsof the pillars may cause problems in the electrical connections. FIGS.26 and 27 show alternative ways of ensuring that no curable polymerhardens on the top surfaces of the via contacts.

In FIG. 26, the substrate 18 would be substantially transparent,allowing the curing with light to occur through the substrate. Anyresidual polymer on the top surface 60 of the pillars would notcross-link, and therefore not harden, because the pillars themselveswould block the light from reaching that portion of the polymer. Thiswould allow the residue to be removed with solvents.

In FIG. 26, the mold 14 shown in several previous figures could beconductive, such as carbon filled silicone. If the pillars areconductive, one could determine the quality of the electrical contactbetween the mold and the pillars before curing the polymer by measuringthe electrical current flow from the mold 14 to the substrate 18. Acapacitive measurement of the capacitance between the mold and thesubstrate may also be used. Good electrical contact would indicate thatlittle or no residue remains on the tops of the pillars. The resultingstructure is shown in FIG. 27.

In this manner, vias allow electrical connections to be made between afirst printed circuit and a second printed circuit, or to expose areasof the first printed circuit. These vias are formed in an additiveprocess, rather than involving etching or removal processes.

It will be appreciated that several of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations, or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. A method, comprising: forming a first active electronic layer on asubstrate; printing an array of conductive pillars on the activeelectronic layer on a substrate; dispensing a curable polymer on thearray of conductive pillars; molding the curable polymer by contactingthe array of pillars with a mold structure to displace the curablepolymer from the upper surfaces of the conductive pillars; curing thecurable polymer to produce a hardened polymer; and forming a secondactive electronic layer on the hardened polymer such that the secondactive electronic layer is in electrical connection with the firstactive electronic layer through the conductive pillars.
 2. The method ofclaim 1, wherein printing an array of conductive pillars comprisesprinting one of metal pillars, polymer bumps covered with a conductivematerial, conductive pillars mixed with structural particles of anothermaterial.
 3. The method of claim 1, wherein molding the curable polymercomprises contacting the array of conductive pillars with a conductivemold to determine if any residual polymer is on a surface of theconductive pillars.